Launchd is not passing arguments to the PHP script

Not sure what happened here, but I got it working now. AbandonProcessGroup definitely needs to be set to true. Also make sure no other errors exist. I had some error, and after clearing that out things started working. It helps by specifying log output files in the launchd jobs. Use StandardOutPath key for log output, and StandardErrorPath key for error output. I specified separate files for error and log output.

The Program key specifies the file to execute, & the ProgramArguments key specifies the arguments which will be passed to the executing process. Strictly speaking you can pass whatever arguments you want to a process, but the convention is that the first one should be the name by which the process was invoked, so most programs ignore their first argument.^‡ The file to execute is obviously necessary information, but if the Program key is is missing, launchd pretends it has the same value as the first argument in ProgramArguments purely as a convenience.

Your first example starts boinccmd & gives it arguments that would be equivalent to the terminal command

--host\ localhost --passwd\ gobbledygook --project\ http://setiathome.berkeley.edu/\ update

which tells boinccmd that you invoked it as "--host localhost" & only passed it two weird arguments.

Your second example separates the arguments correctly, but as Eddie Kelley suggested it needs one inserted at the front. It tells boinccmd that you invoked it as "--host", then passed another six arguments. boinccmd can recognise the last five as being two options, but has no idea what the "localhost" business is about. As far as boinccmd can tell, it was invoked from the terminal as

/Library/Application\ Support/BOINC\ Data/boinccmd localhost --passwd gobbledygook --project http://setiathome.berkeley.edu/ update

(note the missing "--host").

boinccmd is probably one of the great majority of programs that don't care what their first argument is, so you could probably actually just shove <string>HELLO</string> at the head of the ProgramArguments array, but it's probably cleaner to remove the Program key altogether & just use this:

<key>ProgramArguments</key>
    <array>
        <string>/Library/Application Support/BOINC Data/boinccmd</string>
        <string>--host</string>
        <string>localhost</string>
        <string>--passwd</string>
        <string>gobbledygook</string>
        <string>--project</string>
        <string>http://setiathome.berkeley.edu/</string>
        <string>update</string>
    </array>

^‡ It can seem like a meaningless redundancy, but some programs use this to good effect: bash et al. act as login shells if their first argument begins with -, & Vim enters various emulation modes if its first argument is ed or vi instead of vim.